Normal human has horizontal vision field of view (FOV) when his/her head and eyes are fixed is about 160° with one eye and is approximately 190° with two eyes. This is schematically depicted in FIG. 1. The vertical FOV is about 50° up and 80° down.
In FIG. 2 is shown binocular horizontal region seen by both eyes simultaneously. This region is confined by an angle of about 130° and it is responsible for depth perception.
The normal way the human eyes stare at an object is rather complicated. The eye movements are governed by six extra ocular eye muscles. The eye various movements must be performed by both eyes in a coordinated manner to ensure proper fixation and depth perception. The main movements are:                Saccadic movement & Smooth pursuit        Microsaccades        Vergence movement        
Below those movements are explained in more details.
Saccadic movements allow a person to look at the desired object. The staring is composed of both eye movements and eyes rotation into the proper gaze direction.
As seen in FIG. 3 the head-fixed horizontal saccades can have amplitudes of up to a total of 130° (75° temporal direction and 55° nasal direction) but in normal conditions the saccades are far smaller, and any shift of gaze direction larger than about 20° is accompanied by a head movement. During such gaze saccades, first the eye produces a saccade to get gaze on a target, whereas the head follows more slowly and the eyes roll back to keep gaze on the target.
For saccadic amplitudes of up to about 60°, the velocity of a saccade linearly depends on the amplitude (the so called “saccadic main sequence”). For instance, 10° amplitude is associated with a velocity of 300°/sec, and 30° amplitude is associated with 500°/sec. In saccades larger than 60°, the peak velocity starts to plateau (nonlinearly) toward the maximum velocity attainable by the eye.
Smooth pursuit eye movements allow the eyes to closely follow after a moving object.
Micro saccades are a kind of fixation, tiny and rapid eye movements having usually amplitudes of up to 2°. They are small, jerk-like, involuntary eye movements, similar to miniature versions of voluntary saccades. The role of micro saccades in visual perception has been a highly debated topic which is still largely unresolved.
A vergence is the simultaneous movement of both eyes in opposite directions to obtain and maintain single binocular vision. When a person looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an close object, the eyes rotate towards each other (convergence), while for looking at a distant object they rotate away from each other (divergence).
An example of the angle δ by which the eyes have to rotate “in” for a near distance to the object e.g. of 350 mm is shown in FIG. 4. This angle δ is calculated as Arctg δ=IPD/2×D, where IPD is interpupillary distance and D is distance to the object. In the situation depicted in FIG. 4 the IPD is assumed 65 mm and D=350 mm.
Thus one can readily appreciate that extra ocular muscles should be able to displace the eyes in a very complex and at the same time coordinated manner in order to ensure normal visual perception. It can be readily appreciated that any disturbance in movement of at least one eye would be associated with misalignment resulting in visual impairment.
In a further disclosure the following terms will be used:                The term “leading eye” refers to a normal, healthy eye. In case of alternating strabismus, the leading eye will be determined by the processing unit.        The term “non-leading eye” or “tilted eye” refers to an eye that has a problem—for example, partial or full immobility resulting in that it is not staring at the desired direction and cannot be displaced in a coordinated manner with the leading eye.        The term “wide-angle camera” refers to a video camera with a relatively short focal length that permits an angle of view wider than approximately 70°.        The term “digital camera” refers to a video camera that is equipped with a solid state sensor (CCD or CMOS or other sensor) that enables capturing and providing the captured images in digital form.        The term “eye tracking” refers to the process of measuring either the point of gaze (“where we are looking”) or to the motion of an eye relative to the head.        The term “eye tracker” refers to a device for measuring eye positions and eye movement.        The terms “gaze”, “stare” and their conjugates could be used alternately and mean “to look at an object of a scene”        